FIG. 2 is a schematic diagram illustrating a prior art OEIC reported at the meeting of the Applied Physics Society, in autumn 1990. In the OEIC, InP series material are used for an optical device and GaAs is used for an electronic device. The devices comprising different kinds of materials are integrated on a substrate comprising GaAs.
In the figure, reference numeral 1 designates a GaAs substrate. An optical element (optical device) 7 comprising an InP series material diode includes a p type region 71 produced by ion implantation of such as Mg ions, p side electrode 72 and, an n side electrode 73. An FET 8 includes an active layer 81 of several thousand angstroms thick produced by ion implantation of such as Si, a gate electrode 82, a source electrode 83 and a drain electrode 84. In addition, the p side electrodes 72 of the optical device 7 and the gate electrode 82 of the FET 8 are connected by wiring 9.
The layer 2 of the InP series optical element 7 comprises a light absorption layer 21 about 5 micron thickness and a buffer layer 22 about 5 microns thick disposed between the substrate 1 and the light absorption layer 21 so as to relieve stress due to the difference in lattice constant between the GaAs substrate 1 and the light absorption layer 21. Accordingly, when the InP series optical element 7 is produced on the. GaAs substrate 1, a mesa about 10 micron thick is formed on a part of the substrate 1. On the other hand, the FET 8 usually has a planar element structure because the active layer 81 is produced by such as ion implantation on the GaAs substrate 1. Therefore, a step of about 10 microns is formed in the device which has the InP series optical device 7 and the FET 8 on the same substrate, because of the difference in height between the InP series optical device and the wafer surface.
A description is given of the operation hereinafter.
When light is incident on the optical device 7, charge carriers are generated by photovoltaic conversion and the optical current due the carriers is applied to the gate electrode 82 of the FET 8 via the wiring 9. Thus, the current flowing between the source electrode 83 and the drain electrode 84 of the FET part 8 is controlled.
In the OEIC produced by the prior art method as described above, since the InP series optical device and the FET which are produced on the same substrate make a step, the photoresist is discontinuous at a corner of the step and the photoresist is not uniform in thickness on the surface of the substrate region which is to be the FET during the production process of the wafer. Thus, the mesa prevents production of a fine pattern. In addition, due to the difference in height, it is impossible to focus light on both the surface of the GaAs substrate and the surface of the InP series crystal at the same time during pattern transfer. This makes it difficult to carry out photolithography processing such as patterning of the photoresist.
Such difficulty in the fabrication process can be solved by embedding the optical device in the substrate. However, in of producing an optical device embedded in the substrate using a method of producing a well in the substrate and carrying out crystal growth on the entire surface of the well as recited in Japanese Published Patent Application No. 2-125664, it is troublesome to grind the crystal to a predetermined thickness after the crystal growth, and an additional process for maintaining the grinding precision is required. In addition, it is difficult to carry out the crystal growth such that the surface becomes planar because the crystal growth is not planar. Accordingly, it is inevitable that a step as shown in FIG. 2 is produced in producing an OEIC.
In a method disclosed in Japanese Published Patent Application No. 63-90867, to produce a device in which an optical device part is embedded in the substrate without non-planar crystal growth, an optical device is produced in a well in a region whose width is narrower than that of the well. However, in this method, narrow grooves are produced at the both sides of the optical device and problems such as breakage of the photoresist still remain unsolved.